In a paper machine, the caliper and other properties of paper and paperboard are typically continuously monitored as the paper is moving. Gauges are usually fastened to what is known as a measuring beam, in which the gauges continuously move in a reciprocating manner in the transverse direction of the paper and continuously measure the paper as it moves forward. Sensors are arranged as close to the surface of the paper as possible to improve measuring accuracy. This causes a plurality of problems, e.g. if the measuring sensors for some reason come into contact with the surface of the paper, the sensors may leave tracks on the paper. Furthermore, the contact may cause fouling of measuring heads, resulting in impaired measuring accuracy in the long run. Sensors contacting the irregularities of the surface of the paper may also cause holes to be formed in the paper. Consequently it is known to use what is known as an air bearing, the sensors being provided with a bored hole or holes through which air is blown in between the sensor and the paper web. Owing to the air bearing, the sensor is able to stay clear of the web, and hence e.g. the irregularities on the surface of the paper do not come into contact with the measuring head. This avoids formation of holes in the paper. Formation of traces on the surface of the paper is also avoided and fouling of the sensors diminishes. Such an air bearing solution is disclosed in e.g. German Offenlegungsschrift 40 11 646 and in WO 95/30877. However, in an air bearing, the thickness of the air layer between the paper and the sensor is about 50 micrometers. When both sides of the paper comprise sensors which are subjected to an air bearing, the total thickness of the air layer is about 100 micrometers. The thickness of the air layer is considerable with respect to the desired measuring accuracy, which is about 1 micrometers. The effect of a thick air layer on the measuring results is usually compensated for by mathematical proceedings. A sample is taken from each produced paper roll and its caliper is analyzed at a laboratory, whereby the effect of the air layer can be determined. However, this is a dissatisfactory manner since it only reveals a momentary actual caliper of the paper. Furthermore, during the time the sample is being analyzed at the laboratory, and before the result is ready, a large quantity of paper has been produced, the caliper of which can no longer be influenced. Hence this is not a flexible manner to adjust the caliper of a web.
U.S. Pat. No. 5,132,619 discloses an apparatus for measuring the caliper of paper, wherein an air bearing is used to keep the apparatus out of contact with the paper to be measured. The sensor, in turn, is arranged to move against the paper by pressurized air. Consequently, the sensor leaves a mark in papers of a more sensitive quality in particular. The sensor also very easily gets fouled. Furthermore, the structure of the equipment is complex and laborious.
It is the object of the present invention to provide a method and an apparatus which avoid the above drawbacks.